Changing password does not handle errors properly in UI

When there is a policy password such as REGEX against which the password is tested, trying to change the password through the UI with an incorrect password is not correctly handled by UI.

We should instead:

1. Notify the user (either administrator or the user itself trying to change pwd) WITHIN the modal about what is going wrong
2. For regex, we should be able to define an error message to display to the end-user.

Reproductions steps for a user changing his password:

1. Add the following XML as a contribution toward a Studio project
2. Log in with a user
3. Go to user settings/Change password
4. Try a password not allowed by the REGEX, for example: "password"
5. Click on "Save changes"
6. The modal is being closed and a toast displays "An error occured while changing password"

Reproductions steps for n administrator changing another one password:

1. Add the following XML as a contribution toward a Studio project
2. Login with Administrator user
3. Go to Administration/User&Groups
4. Select a user
5. Click sur "Change password"
6. Try a password not allowed by the REGEX, for example: "password"
7. Click on "Save changes"
8. The modal is being closed without displaying an error

<extension point="userManager" target="org.nuxeo.ecm.platform.usermanager.UserService">
    <userManager>
        <userPasswordPattern>(?=.{9,})(?=.*[A-Z])(?=.*[a-z])(?=.*[0-9])(?=.*\p{Punct}).*</userPasswordPattern>
    </userManager>
</extension>